The starting point is an austenitic steel whose alloying constituent quantities are largely standardized, e.g., steel carrying the German Stock Number 1.4550 which require a carbon content under 0.1%, a niobium content higher than the eight fold of the carbon content, as well as a chromium content of 17 to 19 wt. %, and a nickel content from 9 to 11.5 wt. %. Impurities level limits are set at 2.0% Mn, 1.0% Si, 0.045% P and 0.03% S by weight.
The properties of iron are modified by the prescribed amounts of the alloying components with the upper limits on impurities dictated by the specified application zone. Higher impurity limits are generally allowed to make it possible to manufacture alloys from standard, inexpensive source materials which conform to commercial impurity standards. The upper limits of many impurities are the result of optimized manufacturing processes. Concentration limits on other alloying constituents are determined through the optimization of pertinent material properties. Steel qualities 1.4301 and 1.4401, for example, contain niobium as an impurity, but otherwise correspond to the usual impurities of 1.4550 steel. In the U.S., the corresponding steel qualities approximately correspond to markings AISI types 348, 304, and 316.
The microstructure of these materials depends upon their composition, thermal treatment and other procedural steps during the manufacturing process. If for example, the material is subjected to high temperatures for extended periods, large grains will form. Impurities and/or the use of lower temperatures during manufacturing discourages grain growth. The formation of coarse grains can be promoted in some cases during forging, where extensive deformation of grains at elevated temperature causes larger grains to be formed when the forging cools. These grains can be reduced through recrystallization. Grain structure affects material properties such as ductility and strength.
Austenitic steels distinguish themselves from other steels because they have suitable mechanical properties while simultaneously possessing a high level of stability in the face of general corrosion, the even removal of material from the surface of a component, a fact which led to early use of austenitic steels as the material of choice for high stress nuclear reactor internal structural components. Industry experience and laboratory testing has show that these materials fail when exposed to low stress, a matter which can be traced back to selective corrosion at grain boundaries ("intergranular stress corrosion cracking", IGSCC). This selective attack on the grain boundaries can be examined outside the reactor in laboratory tests ("outpile test") by conducting corrosion tests under special aggressive conditions. The results of such tests, show that austenitic steel which is resistant to IGSCC when not exposed to radiation, does fail during inpile testing where radiation is present. The in-reactor failure mechanism is therefore called "irradiation assisted stress corrosion cracking ("IASCC"). It is suspected that phosphorus and silicon are forced to the grain boundaries leading to a susceptible site for the onset of corrosion. Supported by outpile IGSCC tests, the articles "Behavior of Water Reactor Core Materials with Respect to Corrosion Attack" by Garzarolli and Rubel and Steinberg's "Proceedings of the International symposium on Environmental Degradation of Materials in Nuclear Power Systems--Water Reactors", Myrtle Beach, S.C., Aug. 22-25, 1983, Pages 1 through 23, recommend that the silicon content be maintained under 0.1 wt. % and the phosphorus content be kept under 0.01 wt. %, while pointing out that irradiation in a reactor enhances the occurrence of selective corrosion.
In "Deformability of Austenitic Stainless Steel and Ni-Base Alloys in the Core of a Boiling and a Pressurized Water Reactor", Proceedings of the 2nd International Symposium on Environmental Degradation of Materials in Nuclear Power Systems--Water Reactors, Monterey/Calif., Sep. 9-12, 1985, Pages 131 to 138, Garzarolli, Alter and Dewes report results from inpile tests that provide some insight into the influence of phosphorus, silicon, and sulfur impurities on IASCC. Standard steel qualities of stock numbers 1.4541, AISI 316 and 348, were subjected to annealing temperatures of 1050.degree. C. and then cold worked approximately 10%. A chemical analysis was performed to determine alloying constituents for each standard to be tested. AISI 348 steel samples had a silicon and phosphorus content (0.59% and 0.017%, respectively). This was lowered, for use as additional samples of "clean" AISI 348, to 0.01% and 0.008% by a special cleaning procedure. The sulfur content was not analyzed but the remainder of this "clean" steel was composed of 0.041% C, 11.1% Ni, 17.7% Cr, 1.65% Mn and 0.76% Nb+Ta by weight. Temperatures used during the annealing processes that followed the cold work were not closely monitored, but did not in any case exceed 1040.degree. C., yielding a grain size of ASTM No. 9.
The sample with the lowest impurity content showed a considerably reduced corrosion rate during outpile tests. Tubes made of the two types of AISI 348 steel were filled with a ceramic that expands when exposed to irradiation, for inpile tests. These tests showed that only the cleaner material remand relatively undamaged with a diametrical-swelling of 0.7% and even 1.4% following irradiation.
Follow-on tests with newly manufactured tubes showed that these positive results occurred at random and could not be reproduced. The factors and parameters obtained coincidentally during the aforementioned successful tests, which could not be replicated or controlled, obviously have an influence on IASCC.
The nuclear industry has learned from its experience with zirconium alloys, that oxygen causes embrittling and a higher incidence of corrosion. It is suspected that nitrogen has a similar influence on austenitic steel, and it was recommended that austenitic steels be used which contain from 0.025% to 0.065% carbon and 1.5 to 2% manganese, which then show a maximum content of 0.03% N, 0.005% P, 0.05% Si and 0.005% S (U.S. Pat. No. 4,836,976).
Long term reactor tests show, however, that the use of these or similar materials, i.e., P, S, N and Si reduced, could not attain the ductility and resistance with regard to IASCC in individual tests. Systematically varying the N-content did not show any particular influence on the impurity content. All clean variants failed during inpile tests, which means that the previously found high resistance for the aforementioned one-time material must be considered coincidental, whose cause lies in the random, unavoidable variations of the composition and/or manufacturing processes.
The exact mechanisms and contributing factors to IASCC as well as the suitable measures for its avoidance are largely unknown because of the rather extensive list of possible influences, longer reactor testing periods, and substantial cost associated with a comprehensive test series. The task of manufacturing tubes for absorber elements or other structural components for reactor irradiation zones out of a suitable austenitic steel, that are sufficiently resistant to IASCC and can be exposed to the stress of long term reactor operation, still remains unfulfilled. This invention is the key to finding the solution to this task.